The present invention relates to one-dimensional image sensors in which the image of a handwritten, typewritten or printed original document is converted into a time series electrical signal.
At the present, for long distance transmission of images of ordinary business papers, a typical signal transmitting device is constructed of photoelectric conversion elements such as PDA's (photodiode arrays) or CCD (charge coupled device) image sensors.
FIG. 1 is a basic circuit diagram of a PDA which is commercially available. The PDA, as shown in FIG. 1, includes photodiode PD.sub.1, PD.sub.2, . . . and PD.sub.n, MOS FET switches S.sub.1, S.sub.2, . . . and S.sub.n, a load resistor R, and a DC source V. The switches S.sub.1, S.sub.2, . . . and S.sub.n have gate electrodes V.sub.G1, V.sub.G2, . . . V.sub.Gn, respectively. As is well known in the art, the light and dark areas picture elements determine the presence or absence of voltages applied to the gate electrodes. The supply of the gate voltage signals is controlled by clock pulse signals on lines 1 and 2 and a shift register S.sub.R.
The operating principle of the PDA of FIG. 1 will be briefly described. This discussion applies also to an image sensor according to the invention as described in detail below. FIG. 2 is an equivalent circuit of the PDA for a single picture element. An optical switch PD and a pn junction capacitor represent the equivalent circuit of a photodiode. In FIG. 2, reference character SW designates a scanning MOS FET switch and reference character R a load resistor.
When a reverse bias voltage V is applied to the photodiode PD with the switch SW closed, the pn junction capacitor C.sub.j is charged to a level Q=C.sub.j V. When the PDA is irradiated by light having a brightness L for a period of time T with the switch SW opened thereafter, an amount of charge Q'=ILT is discharged from the capacitor C.sub.j where the sensitivity of the photodiode is I A/lux. When the switch SW is closed to again supply the voltage V, the capacitor C.sub.j is charged to Q' again. The recharging current flows through the resistor R producing a voltage drop which forms the image signal. The above described operation is repeatedly carried out to provide a continuous time-series signal.
The above-described PDA or CCD image sensor is made up of high density picture elements. The array length is typically very short, usually not more than 30 mm. Therefore, a PDA or CCD image sensor cannot be used to read ordinary clerical or business papers without employing a contracting optical system 11 as shown in FIG. 3. Therefore, a conventional PDA or CCD image sensor is necessarily bulky. In FIG. 3, reference numeral 10 designates an integrated image sensor such as a PDA or CCD image sensor and reference numeral 12 an original document.
In order to eliminate the above-described difficulty, a reading device as shown in FIG. 4 has been proposed in the art. In this device, a photoconductive film light receiving element 6 extending over the width of an original and an integrated drive circuit element 16 are employed as an image sensor while a self-focusing lens 14 is employed as 1:1 image forming optical system. The image sensor 13 is illustrated in FIG. 5 in detail. In FIG. 5, reference numeral 17 designates an insulating substrate, 18 belt-shaped electrodes each having a width corresponding to a picture element size, 19 a photoconductive film, 20 a light transmissive electrode, 21 and IC chip including drive circuit elements 16 such as switching elements (corresponding to the switches S.sub.1, S.sub.2, . . . S.sub.n in FIG. 1), 22 connecting leads, and 23 wire bondings. In this device, the dimensions of the light receiving element section are relatively large. However, the overall size is much smaller than that of the system shown in FIG. 3.
A device of this type is nonetheless disadvantageous in that wire bondings are required to connect the leads of the IC chip to the external leads on the substrate 17 as shown in FIG. 5 thus requiring a great deal of manufacturing time so that the device is accordingly expensive. Furthermore, as it is necessary to connect the IC chip to the conductors on the insulating substrate with thin wires or with the use of a thermal pressure adhesion technique, the manufacturing yield of the device is relatively low and, accordingly, the device is high in manufacturing cost and low in reliability.
Accordingly, a first object of the present invention is to provide a one-dimensional image sensor which is simple to manufacture and high in manufacturing yield.
A second object of the invention is to provide a one-dimensional image sensor large in size, low in manufacturing cost and free from mechanical wiring.
A third object of the invention is to provide a one-dimensional image sensor in which optical sensitivity distribution such as for instance complementary for inadequate blue light sensitivity, as is peculiar to a PDA or CCD image sensor, is readily achieved.
A fourth object of the invention is to provide a one-dimensional image sensor large in size thereby permitting the use of a self-focusing lens optical system so as to reduce the overall size of a reading device using the invention.